You can include the addendum, but don't expect it to do much good. Since you don't have a real reason for your bad grades, all you can focus on is putting distance between yourself and your UG GPA. Solid work experience might help a little at some schools. Don't expect your study abroad grades or graduate courses to help, adcoms will consider those grades inflated. It's usually a lot easier to get A's in grad courses or study abroad programs than in undergraduate courses in the US. Your best bet is to bring up your LSAT score as much as possible.

I've seen this sentiment cropping up more and more here, and I want to stop it in its tracks.

Explanatory essays/addenda CAN hurt. Very much sometimes.

If you come across as making excuses, it will hurt. If you come across as entitled, it will hurt. If you come across as bitter at the education system, it will hurt. If you write an addendum that's unnecessary, it will hurt (it shows you lack perspective).

And it doesn't have to be the whole essay - one poorly phrased line (or even clause) can change the tone of an essay from explanatory to whiny.

If there's a question that needs answering, write an explanatory essay. But there are not many questions that NEED answering. If you're writing for any other reason, then you risk coming across as making excuses. And there's very little benefit, as without a medical (or death-in-the-family) situation, it's probably not going to make much of a difference. Even with those explanations, it probably won't make much difference.

To the OP - there's really not much to say. You were a middling student in undergrad. It happens. Your more-recent transcripts will show that you can do better in an (inflated) academic program, and your work experience will hopefully speak for itself. If you write about how these things demonstrate your maturity, you'll just be telling them something they already know.

Unless you did something awesome since college. If that's the case, then you can write the essay.